One 、 Simulate the implementation of character functions （strlen,strcpy,strcat,strcmp）

（1） Simulation Implementation strlen

#include<stdio.h>
int my_strlen(char* p)
{
    
	int num = 0;
	while(*p!='\0')
	{
    
		num++;
		p++;
	}
	return num;
}
int main()
{
    
	char arr[] = "abcdef";
	int ret = my_strlen(arr);
	printf("%d", ret);
	return 0;
}

（2） Simulation Implementation strcpy

#include<stdio.h>
#include<assert.h>
void my_strcpy(char* dest,const char* src)
{
    
	assert(dest && src);
	
	while(*dest++=*src++)
	{
    
		;
	}
	
}
int main()
{
    
	char arr1[] = "abcddd";
	char arr2[10] = {
     0 };
	my_strcpy(arr2, arr1);
	printf("%s\n", arr2);
	return 0;
}

（3） Simulation Implementation strcat

char* my_strcat(char* dest,const char*src)
{
    
	char* ret = dest;
	while(*dest)
{
    
	dest++;
}
	while (*dest++ = *src++)
		;
	return ret;
}
int main()
{
    
	char arr1[10] = "abcd";
	strcat(arr1, "zhu");
	printf("%s\n", arr1);

	return 0;
}

（4） Simulation Implementation strcmp

int my_strcmp(char* str1,char* str2)
{
    
	assert(str1 && str2);
	while (*str1 == *str2)
	{
    
		if (*str1 == '\0')
			return 0;
		str1++;
		str2++;
	}
	if (*str1 > *str2)
		return 1;
	else
		return -1;
}
int main()
{
    
	char arr1[] = "abcd";
	char arr2[] = "abcd";
	int ret = my_strcmp(arr1, arr2);
	printf("%d", ret);
	return 0;
}

Two 、 Simulate the implementation of memory functions （strstr、memcpy、memmove）

（1） Simulation Implementation strstr

char* my_strstr(const char* str1, const char* str2)
{
    
	assert(str1 && str2);
	const char* s1 = str1;
	const char* s2 = str2;
	const char* p = str1;
	while(*p)
	{
    
		s1 = p;
		s2 = str2;
		while(*s1==*s2&&*s1!='\0'&&*s2!='\0')
		{
    
			s1++;
			s2++;
		}
		if(*s2=='\0')
		{
    
			return p;
		}
		p++;
	}
	return NULL;
}
int main()
{
    
	char arr1[] = "zhucheng666";
	char arr2[] = "en";
	char* ret = my_strstr(arr1, arr2);
	printf("%s\n", ret);
	return 0;
}

（2） Simulation Implementation memcpy

my_memcpy(void* dest,const void* src,int num)
{
    
	void* ret = dest;
	while (num--)
	{
    
		*(char*)dest = *(char*)src;
		dest = (char*)dest+1;
		src = (char*)src + 1;
	}
	return ret;
}
int main()
{
    
	int arr1[] = {
     1,2,3,4,5 };
	int arr2[10] = {
     0 };
	my_memcpy(arr2, arr1, 8);
	int i = 0;
	for (i = 0; i < 2; i++)
		printf("%d ",arr2[i]);
	return 0;
}

（3） Simulation Implementation memmove

void *my_memmove(void* dest,const void*src,size_t num)
{
    
	assert(dest && src);
	void* ret = dest;
	if(dest<src)
		// front -> after 
	{
    
		while (num--)
		{
    
			*(char*)dest = *(char*)src;
			dest = (char*)dest + 1;
			src = (char*)src + 1;
		}
	}
	else
	{
    
		while (num--)
		{
    
			*((char*)dest + num) = *((char*)src + num);
		}
		return ret;
	}

The above is the implementation principle of the most basic character functions and memory functions , You can learn about , A deep understanding , Only in this way can we make better use of it ！